The 'square' O-rings can leak. I've replaced them in 2 engines that leaked a LOT from them. They tend to curl inward as the head is torqued down.
Use Parker #02-111 O-rings, which are 2.62 x 10.77mm in size. These are thick enough to cope with the thicker-than-OEM head gasket you now have, which WILL cause an oil leak at the cylinder-head joint if you use normally-sized O-rings.
If you can't find them, PM me for a pair.
Are they nitrile orings or did you find Vinton?
These are Buna 70. Viton isn't needed in these engines, they were designed using conventional Buna type.
In the lower side of the cylinders there's 2 O-rings, also, and in the later (K4 onward) engines these became silicone type. In the early engines they were Buna, too. There, either type works fine.
Viton is useful when fuel is encountered, but only if the O-ring is correctly sized, as Viton doesn't seal quite as completely as Buna at low pressures.
This is due to the ethanol having been added to our gas AFTER this design existed: you sometimes find Viton in the more modern bikes' fuel systems (1980s onward) to cope with the MTBE and ethanol.
In these bikes, upsizing the O-ring in the fuel tees and pipes at the carbs to a little thicker and switching to Viton may have some [theoretical] benefit. In practice, though, unless the fuel pipes/tees are often disconnected from the carbs while 'wet' with gas, there isn't a significant advantage, save one instance: in the 750 sandcast and early K0 carbs there's just 1 O-ring on each side of the fuel tees instead of the 2 used after that. Using Viton there (upsize the cross-section from 1.9mm to 2.0mm if you do (making them 2x7.9 or 2x8mm size), because it will require that extra pressure to make the Viton seal. The downside of this swap is important to know: the Viton is a harder, stiffer rubber, and by increasing the thickness to make it seal well, it also GREATLY increases the pressure of the O-ring against the inside of the carb bodies where those tees press in - this can overstress the old zinc-aluminum (Zamac) castings and make it split the carb's fuel port (K0 carbs are hard to find!). So, it may not be worth the risk, just to ensure that fuel might not weep a wisp for a few minutes when turning on the gas after the carbs have fully dried out and the fuel lines dried to empty from sitting a long time.
Buna70 will swell about 3% to 5% when wetted with our modern gas or synthetic oils. After long-term exposure to ethanol the Buna will also shrink about 2% when dried back out from this soaking, but it re-expands more than that to a slightly-more-than-original cross-section with O-rings.
